Diving with Sea Lions

The Playful Puppies of the Sea

Few marine creatures seem to delight so much in the joy of life as a sea lion. Watching them play with each other, with divers and with other marine creatures, it is hard not to imagine they are simply having fun. These pinnipeds (flippered feet) apparently delight in performing a series of aquabatic stunts much to the amusement of divers. They can roar past underwater at break-neck speed then twist and turn with seemingly effortless ease.

In the marine world where sex, food and violence are life's staple diet, sea lions provide a little comic relief to Tennyson's "nature, red in tooth and claw". Or at least they appear to, until we consider what ruthless predators they can be.

Sea Lion Fact Sheet

They belong to the own subfamily 'Otariinae'. Of their family group, they are the most frequently encountered by scuba divers and include the Galapagos and California sea lion.

Distinguishing Features

Sea lions are distinguishable from other similar marine mammals by external ear flaps, the ability to walk, and indeed run, on all fours and short thick hair.

They are in the 'Otariidae' (eared seals) family along with earless fur seals. Recent genetic discoveries have eliminated a long-held distinction between the subfamilies of seals and sea lions, since it was discovered that the northern fur seal is more closely related to sea lions than to other fur seals. In comparison to a true seal, a sea lion has coarser, shorter fur, greater bulk and attacks bigger prey. The larger species can grow to 3m (10 feet) and weigh up to 1,000 kg.

They have excellent underwater vision and when you encounter them in the water you can see they have a large transparent covering over the eye which can reflect the light like a dive mask.

Behaviour

Sea lions are intelligent mammals and this intelligence has been used both to entertain and for more serious purposes. The 'performing seal' you see at circuses and water parks, balancing balls and clapping, is almost always a sea lion. They have also been trained by the U.S. military to 'detain' scuba divers.

In the wild, they tend to inhabit sandy beaches and gently sloping, rocky shorelines. Galapagos sea lions are so unafraid of man they can also be seen on jetties, moored boats in a harbour and even on park benches! They are highly thigmotactic (seeking body contact) and often loaf around in piles on the beach.

Galapagos sea lions and to a lesser extent, their Californian cousins, are very playful and likely to approach divers closely. They often bring themselves face to face with wide-eyed scuba divers. They can swim at speeds of up to 40 km/h (25 mph) so it is impossible for humans with clunky dive gear to try to follow sea lions. Better to stay still and let them zoom around. They have been known to imitate diver behaviour by staring at a diver and then blowing bubbles!

They are also playful with each other, whether playing tag, or using a prop such as a starfish, red-lipped batfish or pufferfish as a toy. However, the poor 'toy' does not seem to share in the fun. Sea lions appear not to know their own strength and have been known, when playing with marine iguanas, to accidentally break their backs.

Attacks on humans are rare and almost always associated with the presence of dead fish in the water. They are happy to accept this easy prey. If it happens to be in the vicinity of a spear fisherman then trouble could ensue. In San Francisco there have been recent reports of swimmers being bitten on the legs by large aggressive males. It is likely that these were territorial acts or actions to protect young pups.

What is certain is that males should be avoided, particularly when there are any acts of aggression on display. It is mostly females that interact with humans. As with dolphins, there have been reports of sea lions coming to the aid of human beings in difficulty in the water, keeping them afloat until help arrives.

Feeding Habits

Fierce predators, they prey on all sorts of fish including groupers, snappers and grunts, and can eat up to 5-8% of their body weight at a time. Galapagos sea lions also eat creole fish and salema in large quantities, although their favourite food is sardines and pollock. They can dive up to 180m (600 feet) in search of food, and can stay underwater for up to 40 minutes.

Reproduction

Sea lions have a gestation period similar to that of a human. A mother can take care of a maximum of 2 pups at the same time. They have sex both on land and in water and have been seen in flagrante delicto in Galapagos on the shore, in the belly-to-belly, missionary position.

Males are much bigger than females which helps them win the sexual approval of the females. In reproduction periods, there would be an inherent risk of getting attacked by one of the many adrenalin-filled bulls confusing you with an intruder.

After copulation, there is a system of delayed implantation, where the fertilised egg floats in the uterus for about 2 months prior to implanting. Normally it is a single pup that is born, after which the mother will stay with it for the first week and bond with it using methods of smell, touch and vocalisation. The mother will them begin to forage by day and return in the evenings to suckle the pup.

Life Cycle

Pups feed on milk at an early age and learn to swim at about 1 or 2 weeks old in shallow, rocky pools in safe, nursery areas. At about 5 weeks old the pup will lose its baby coat, or lanugo, and start to take on the appearance of a small adult sea lion. Self-sufficiency begins to develop when the pup begins to feed inshore for itself but it may take considerably longer before it does not rely at all on its mother.

For the first 2 years or so, pups will only feed in the open water with their mother, thereafter feeding alone. Female pups often stay in the same harem when they are grown adults. Alpha males usually don't last long because they are so busy maintaining and servicing their harem that they lose weight and the ability to defend themselves against younger stronger males. So a female pup may grow up in the same harem with a steady supply of different alpha males at its head.

When they are 7 years old, males are expelled from the group by the alpha male and go off in search of their own harem. As the male ages, the bulbous curve on his head becomes increasingly pronounced, so an old bull can be identified by his large, humped head.

Sea lions have an average life span of 20-30 years.

Predation

Sea lions, particularly when young, are vulnerable to attacks by sharks and killer whales (orcas). The bull male is very protective over his harem and young pups, and will react aggressively to any perception of danger. Many young will still fall prey to the local shark population. In Galapagos their main threats are the Galapagos shark and blacktip sharks.

Distribution

They are be found in many parts of the world particularly in the tropical and subarctic regions. The one species known to have become extinct in the 1950s is the Japanese sea lion (zalophus japonicas). Genus names of extant species include the following:

Australian sea lion (Neophoca cinerea)

South American sea lion (Otaria flavescens)

New Zealand sea lion (Phocarctos hookeri)

Californian sea lion (Zalophus californianus)

Galapagos sea lion (Zalophus wollebaeki)

Steller sea lion (Eumetopias jubatus), the largest species, found in the Northern Pacific, especially around Alaska

Ecological Considerations

Dangerous fishing practices pose a threat to sea lions. Long lines and nets have been known to ensnare and kill them as non-targetted by-catch.

They are susceptible to picking up diseases from non-native species. Domestic dogs can transmit distemper to them, a condition which can ultimately kill them as well as spread throughout a population.